The present invention relates generally to improvements in communication receiver circuits. More particularly, the present invention relates to methods and apparatus for high performance reception of radio frequency (RF) communication signals using receiver circuits to dynamically control the gain of a low noise amplifier (LNA) based on received power level.
In general, wireless telephones function as mobile stations which allow a user to communicate within a particular geographic serving area. Continuous communication is provided even as the telephone moves about within the service area. Such continuous service is provided in wireless cellular networks by providing base stations throughout the geographic service area. Each base station provides service to a geographic area called a cell.
An increasingly popular wireless digital communication methodology is code division multiple access (CDMA). CDMA provides a means for organizing radio frequency energy over a range of frequencies and moving among the frequency range on a code divided basis. CDMA systems require receivers with a wide dynamic range. When a CDMA mobile system is at the edge of a cell, the CDMA mobile receiver must exhibit a very low noise figure in order to detect the desired low power signal, such as xe2x88x92104 dBm, amidst the surrounding noise. Yet, when the mobile system is close to the base station, the CDMA mobile receiver may receive signals of much greater strength, such as xe2x88x9225 dBm. These two extremes require that the receiver have both a very low noise figure (NF) and a very high input third order intercept point (IIP3). However, it is not necessary to achieve both requirements simultaneously. For example, at low power levels a low noise figure is essential while only a moderate level of IIP3 is necessary to prevent desensitization of the receiver due to noise. Yet, at high power levels a high IIP3 is needed, irrespective of the noise figure.
Due to these dual requirements, most prior art CDMA receivers incorporate some form of gain control into a low noise amplifier (LNA) of the receiver which changes the receiver from a low noise figure, high gain state to a low gain, high IIP3 state. These prior art approaches, which typically use switches or a digital signal processor (DSP) to change the state of the LNA, suffer from numerous drawbacks. For example, the insertion loss of the switches contributes to the noise figure and degrades performance when operating in the high gain state. Further, the switches allow only two states of gain control to be utilized. Due to the processing delays of the DSP, typically only three states of gain control can be utilized and significant errors can be introduced into the system by the delays. In both of the prior art approaches, current is wasted when the receiver is operated inefficiently due to the limited number of gain control states. Typical prior art systems control the gain based on a baseband signal derived from the received RF signal, resulting in substantial latency when changing the gain.
Accordingly, it would be highly advantageous to provide continuously variable gain control for the LNA of a wireless receiver such that the gain, IIP3 and noise figure are continuously varied according to input RF power levels.
The present invention provides methods and apparatus for providing continuously variable gain control for a low noise amplifier of a RF wireless receiver. The gain, IIP3 and NF of an LNA are continuously varied according to the received power levels, causing the receiver to utilize less current at different power levels. For example, at high gain levels the IIP3 and NF are at a minimum, while at low gain levels the IIP3 and the NF are at a maximum. By continuously varying the gain of the LNA throughout the operational range, the present invention advantageously achieves wider dynamic range and higher power efficiency than typical prior art approaches.
According to one aspect, the present invention includes a power coupler and a power detector which are utilized to produce a rectified voltage which is proportional to the input power or output power of an LNA. The rectified voltage is utilized by a control circuit which produces a signal which controls the gain, IIP3 and NF of the LNA. According to another aspect, diode circuitry is utilized to produce the rectified voltage.
By controlling the gain of the LNA based on the RF signal power level present at the input or the output of the LNA, the present invention advantageously minimizes the delay in controlling the gain, allowing a wireless receiver to react quickly to changes in received signal strength, especially when fading occurs.
A more complete understanding of the present invention, as well as further features and advantages, will be apparent from the following Detailed Description and the accompanying drawings.